The invention relates to a spray element for a spray head, especially for spraying the molds of pressure casting machines. The spray element has a spray nozzle which can be fed with control air and the medium to be sprayed, and a valve is provided with a control piston operated by control air.
DE 32 38 201 A1 discloses a spray element, in which a spring-loaded control piston is forced by the spring force with its one end, on which a gasket sits, against a valve seat in front of which a connecting bore opens to a parting agent connection. Therefore, if the piston is moved against the spring force by controlling air, the valve seat opens and the parting agent that is to be sprayed can issue through a center nozzle opening where it can be sprayed by means of controlling air jets which surround the jet of the parting agent.
A certain disadvantage of such spray elements include that, when the parting agent feed to the nozzle orifice is shut off, i.e., when the gasket of the control piston is driven by control air against its seat, it can lead to unwanted dripping of any parting agent still present around the discharge opening.
An object of the invention is to improve a spray element of the kind referred to above such that any dripping of the medium being sprayed is prevented by comparatively simple means.
To achieve the object, in a spray element of the kind referred to above, the control piston is arranged next to the spray nozzle with both ends of the control piston in the cylindrical sections of a housing. The first end has a smaller diameter than the second end and is guided by a sealing ring in a cylindrical section before a first chamber fed with the medium to be sprayed. The second end is guided in a second cylindrical chamber into which a communicating bore leads into the spray valve and which has on the side facing away from the first end of the control piston a connection for control air. The distance between the two ends of the control piston is chosen such that the sealing ring when in the starting position lies sealingly in the cylindrical section, and when control air is applied to the second end it enters the first chamber.
By this arrangement, therefore, when control air is applied to the control piston, the feeding of the medium to the spray nozzle is released, namely through the second cylindrical chamber, so that then this medium can be sprayed with compressed air. If, however, the control air is not applied, then the control piston is moved by the pressure of the medium being sprayed back to its starting position in which the feeding of the medium being sprayed is stopped. Simultaneously, however, the backward movement of the second end of the control piston increases the volume in the second cylindrical chamber and thus a certain vacuum is produced, with the result that the medium held in the bore connecting to the spray nozzle is moved back, i.e., sucked back so that any after-drip is prevented.
In developing the invention, the housing of the control piston can be provided with an extension to receive the spray nozzle, this extension can also be a one-piece component of the housing.
In a further development of the invention, all connections for control air, the medium to be sprayed, and compressed air can be arranged on the side of the housing that faces away from the discharge opening of the spray nozzle. Thus an easily assembled modular component is achieved which can be attached in a relatively simple manner to corresponding spray arms provided with passages carrying the individual media.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.